Processes for the treatment of sour petroleum distillates wherein the distillate is treated in contact with an oxidation catalyst in the presence of an oxidizing agent at alkaline reaction conditions have become well-known and widely practiced in the petroleum refinery industry. Said processes are typically designed to effect the oxidation of offensive mercaptans contained in a sour petroleum distillate with the formation of innocuous disulfides-a process commonly referred to as sweetening. The oxidizing agent is most often air. Gasoline, including natural, straight run and cracked gasolines, is the most frequently treated sour petroleum distillate. Other sour petroleum distillates include the normally gaseous petroleum fraction as well as naphtha, kerosene, jet fuel, fuel oil, lube oil, and the like.
A commonly used continuous process for treating sour petroleum distillates entails contacting the distillate with a metal phthalocyanine catalyst dispersed in an aqueous caustic solution to yield a doctor sweet product. The sour distillate and the catalyst containing aqueous caustic solution provide a liquid-liquid system wherein mercaptans are converted to disulfides at the interface of the immiscible solutions in the presence of an oxidizing agent--usually air. Sour petroleum distillates containing more difficulty oxidizable mercaptans are more effectively treated in contact with a metal chelate catalyst disposed on a high surface area adsorptive support--usually a metal phthalocyanine on an activated charcoal. The distillate is treated in contact with the supported metal chelate catalyst at oxidation conditions in the presence of an alkaline agent. One such process is described in U.S. Pat. No. 2,988,500. The oxidizing agent is most often air admixed with the distillate to be treated, and the alkaline agent is most often an aqueous caustic solution charged continuously to the process or intermittently as required to maintain the catalyst in the caustic-wetted state.
The prior art shows that the usual practice of catalytically treating a sour petroleum distillate containing mercaptans involves the introduction of alkaline agents, usually sodium hydroxide, into the sour petroleum distillate prior to or during the treating operation. See U.S. Pat. Nos. 3,108,081 and 4,156,641. The prior art also discloses that quaternary ammonium compounds can improve the activity of these catalytic systems. For example, see U.S. Pat. No. 4,290,913 and 4,337,147. In these patents the catalytic composite comprises a metal chelate, an alkali metal hydroxide and a quaternary ammonium hydroxide disposed on an adsorptive support.
Applicants have found that onium compounds in which the cationic atom is selected from the group consisting of phosphorous, arsenic, antimony, oxygen, and sulfur are effective promoters for sweetening sour petroleum distillates. An onium compound is an ion-pair compound containing one or more organic cationic groups in which the positively charged atom is a nonmetallic element other than carbon and in which the positively charged (cationic) atom is not attached to hydrogen. The most common onium compound is the quaternary ammonium compound. See "Nomenclature of Organic Compounds," Advances in Chemistry Series 126, J. H. Fletcher, O. C. Dermer and R. B. Fox, editors, Amer. Chem. Soc. publishers, page 189-90, 1974.
There is no mention in the prior art that onium compounds in which the cationic atom is, for example, phosphorous or sulfur would be effective promoters for the conversion of mercaptans to disulfides. Applicants have found, for example, that phosphonium compounds have better stability than ammonium quaternary compounds. Thus, applicants have provided an improved catalytic composite and process for sweetening a sour petroleum distillate.